The present invention relates to motorized actuators, such as of a type used to operate valves and dampers in a heating, ventilation and air conditioning system; and more particularly to mechanisms for braking such actuators and for detecting when the actuator stalls.
Electrically powered actuators are commonly used to open and close valves and airflow dampers in heating, ventilation and air conditioning (HVAC) systems. These actuators usually include an electric motor which is connected by a gear train to the valve or damper. This allows a low torque motor to operate a relatively large load. The motor can be operated to place the valve or damper in a number of positions between the extreme open and closed limits.
It is important in some installations that the device operated by the actuator assume a fail-safe position in the event that electrical power is lost. For example, it often is desired that a damper in an HVAC duct close when electrical power is lost as may occur during a fire. This ensures that air is not being supplied through the damper to a fire and that smoke is not circulated to other parts of the building.
In order to provide such fail-safe operation, actuators incorporate a return spring which winds as the actuator moves the valve or damper into an open position. Thus in the open position energy to close the valve or damper is stored in the spring. The spring unwinds as the actuator closes the valve or damper. When the motor stops, its detent torque as amplified by the gear train prevents the actuator from moving, thus preventing the return spring energy from moving the actuator into the closed, or home, position. Sometimes after the motor stops, a small current, less than that required to turn the motor, still is applied to further impede the spring from moving the actuator load.
A clutch is provided in the gear train between the motor and the spring with the spring always being connected to the output of the actuator. The clutch is operated by a solenoid so as to be engaged whenever electricity is available for powering the actuator motor, whether or not electricity is actually being applied by a control circuit to the motor. In the event that power is lost to the actuator, the clutch disengages thereby decoupling the motor from the gear train. This isolates the motor detent torque from inhibiting operation of the spring and enables the spring force to move the actuator into the home position.
One of the drawbacks of this type of arrangement occurs upon a power failure when the spring is wound significantly, thus storing a relatively large amount of energy. When the clutch now disengages, a very large spring force is applied, resulting in the output being driven at an excessively fast speed. That action may damage the valve or damper being operated by the actuator.
Another problem occurs when the actuator reaches one of the extreme travel limits of the valve or damper. At that time the valve or damper prevents the actuator motor from turning even though electricity still is being applied. This is known as a stall condition of the motor and the motor can be damaged if electricity continues to be applied for too long a time while the motor is stalled. If a stall condition persists for an excessive period, the gear train and actuator load also can be damaged. As a consequence, sensors are often attached to the motor to detect the stall condition.